...two questions, please!
a. Does that mean 5 mins per month of pavement walking? Does sniffing around tree off the lead, playing with other dogs, mooching around in woods/commons count? If so, I think we're in trouble 
b. At what age does it stop applying, if ever?
Our 6 month old lab cross really enjoys his off-lead time, pottering around, playing, snaffling treats doing a bit of training...the actual walk we do takes 20 minutes if I do it alone, but with him its more like an hour!